Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric material layer between a bottom (first) electrode and a top (second) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs operating at frequencies close to their fundamental resonance frequencies may be used as a key component of radio frequency (RF) filters and duplexers in mobile devices.
An acoustic resonator typically comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
In general, an acoustic resonator comprises different lateral regions that may be subject to different types of resonances, or resonance modes. These lateral regions can be characterized, very broadly, as a main membrane region and peripheral regions, where the main membrane region is defined, roughly, by an overlap between the two plate electrodes and the piezoelectric material with layers structurally attached to each other, and the peripheral regions are defined as areas outside the main membrane region. Two peripheral regions, in particular, are defined as a region located between the edge of the main membrane region and edge of the air-cavity, and a region of an overlap of at least one plate electrode and the piezoelectric material with the substrate. The main membrane region is subject to electrically excited modes generated by the electric field between the two plate electrodes, and both the main membrane and the peripheral regions are subject to certain derivative modes generated by scattering of acoustic energy confined in the electrically excited modes. The electrically excited modes comprise, for instance, a piston mode formed by longitudinal acoustic waves with boundaries at the edges of the main membrane region. The derivative modes comprise, for instance, lateral modes formed by lateral acoustic waves excited at the structural discontinuities located at the edges of the main membrane region and the peripheral regions.
The lateral modes facilitate continuity of appropriate mechanical particle velocities and stresses between the electrically driven main membrane region and the essentially non-driven peripheral regions. They can either propagate freely (so called propagating modes) or exponentially decay (so called evanescent and complex modes) from the point of excitation. They can be excited both by lateral structural discontinuities (e.g., an interface between regions of different thicknesses in the main membrane region, or an edge of a top or bottom electrode) or by electric field discontinuities (e.g., an edge of a top electrode where the electric field is terminated abruptly).
The lateral modes generally have a deleterious impact on the performance of an acoustic resonator. Accordingly, some acoustic resonators include ancillary structural features designed to suppress, inhibit, or mitigate the lateral modes. For example, an air bridge may be formed under the top electrode on the top electrode connecting edge of the acoustic resonator in order to eliminate the transducer effect over the substrate. In another example, a frame may be formed by a conductive or dielectric material within the boundary of the main membrane region to minimize scattering of electrically excited piston mode at top electrode edges and improve confinement of mechanical motion to the main membrane region.
The conventional implementation of these ancillary structural features has a number of potential shortcomings. For instance, depending on their specific design, they may be a source of additional scattering of the piston mode which may outweigh their benefits. Also, some design choices may produce only modest performance improvements while significantly driving up cost. Moreover, the formation of ancillary structural features may degrade structural stability or interfere with the formation of overlying layers.
In addition, conventional FBARs rely on an air interface being present both on bottom and top side of the resonator. In contrast to SMRs, an air interface present on the bottom side of the resonator prevents parasitic acoustic energy leakage to the substrate and therefore improves the overall electrical performance of FBARs without the complexities associated with the design of a wide-band, solid-state acoustic mirror such as Distributed Bragg Reflector. On the other hand, however, lack of a solid connection of the active portion of the resonator with the substrate results in worse heat removal capabilities and weaker structural stability as compared to conventional SMR designs. Accordingly, in view of these and other shortcomings of conventional FBARs, there is a general need for improved acoustic resonator designs that address these issues without compromising the electrical performance of acoustic resonators and filters comprising these resonators.
There is also a need for acoustic resonator designs enabling construction of RF filters that have better insertion loss (IL) performance for particular frequencies or frequency ranges. FIG. 1 is an idealized plot 2 of IL as a function of frequency for an RF filter comprising a typical state-of-the-art FBAR. FIG. 2 is a general, cross-sectional view of a typical state-of-the-art FBAR 5 comprising a bottom electrode 6 disposed on a substrate 7 over an air cavity 8, a piezoelectric material layer 9 disposed on the bottom electrode 6, and a top electrode 11 disposed on the piezoelectric material layer 9. The bottom electrode is terminated with the planarization layer 10. An overlap among the top electrode 11, the piezoelectric material layer 9, the bottom electrode 6, and the air cavity 8 defines a main membrane region of the FBAR 5. Air gaps 12 and 13 exist between the top electrode 11 and the piezoelectric material layer 9 at the edges of the main membrane region. The air gaps 12 and 13 form an air bridge 14 and an air wing 15, respectively. The bottom and top electrodes 6 and 11, respectively, are often made of molybdenum (Mo) and the piezoelectric material layer 9 is often made of aluminum nitride (AlN). With reference to FIG. 1, the right shoulder 3 of the plot 2 is higher and has steeper roll-off than the left should 4 of the plot 2 due to the quality factor QP at the parallel resonance frequency FP of the FBAR 5 being three to four times better than the average quality factor QSW at frequencies below series resonance frequency FS of the FBAR 5. The FBAR 5 experiences degradation of the quality factor QSW for frequencies below the FS due to so-called rattles in the frequency response. The rattles are caused by the excitation of parasitic lateral resonances in the FBAR 5, which detrimentally impact the main resonance of the FBAR 5. A need exists for acoustic resonator devices that provides improved quality factor QSW for frequencies below FS yielding improved left-shoulder IL performance of RF filters comprising such acoustic resonator devices.